In many vehicles which have a hydraulic steering motor and a hydraulic implement circuit which receive flow from a single pump, the steering motor is connected upstream of and in series relation with the implement system. As a result, the pump is working against the sum of the load pressures of the steering motor and the implement circuits. The pump, however, can only operate up to its rated operating pressure and can only supply its maximum rated flow rate. Hence, if the implement circuit is experiencing a high load pressure, only a relatively low pressure (a portion of the pump rated operating pressure) is available across the steering motor for operating it. This can lead to low torque stalling of the steering motor. One can utilize a pump with a very high operating pressure and flow ratings, along with a high power engine to drive the pump, but this adds to the weight, size and cost of the vehicle.
Prior art load sensitive hydraulic systems, on the other hand, suffer from the disadvantage that the return flow from the priority work circuit is passed to tank regardless of the pressure requirements of subsequent circuits. This requires the use of a wastefully large pump.
In essence, it would be advantageous to have a system which gave first priority to the steering motor, which utilized a pump primarily sized to the steering motor, wherein the steering motor was not subject to low torque stalling, which provided both high speed turn and high speed implement correction when operating at relatively lower pressures and prevented such dual high speed operation when the implement was operating at a relatively higher pressure level, and which provided underspeed-type engine protection as well.
The present invention is directed to overcome one or more of the problems set forth above.